1. Field of the Invention
The present invention relates to a radio channel controller and a radio channel controlling method, which are applied to a mobile communication system.
2. Description of the Related Art
In a mobile communication system, restriction on handover or “handover restriction” may sometimes be set for a base transceiver station or for a cell as an area in which the base transceiver station provides a communication service, in order to install a new base transceiver station and perform its accompanying wave measurement and adjustment or to carry out regular or occasional maintenance operation.
Telephone call is allowed only by a mobile communication carrier or by a mobile station for a particular operator serving under the mobile communication carrier, in the cell to which the handover restriction is set. Thereby, confirmation on normality of the telephone call, confirmation on operational parameters, and the like are performed.
General users are not allowed to establish communication by using the cell to which the handover restriction is set.
Herein, the cell in which the handover is being restricted and thus its usage is prohibited may hereinafter be referred to as “use-prohibited cell”.
In a mobile communication system in which the same frequency range can be used for all of the base transceiver stations, such as WCDMA (Wideband Code Division Multiple Access), OFDMA (Orthogonal Frequency Division Multiple Access) and the like, radio waves generated by the base transceiver station under handover restriction become sources of interference. Thus, restriction on construction or “construction restriction” is generally set for the base transceiver station under handover restriction as a cell for construction or maintenance, as far as announcement information and handling in a control station are concerned, although such a base transceiver station under handover restriction is managed as one of the base transceiver stations within a commercial system, as with general base transceiver stations.
By operating as described above, a mobile station currently in a state of waiting for a call is not allowed to wait a call in the cell for construction or maintenance, so the mobile station is prompted to receive a call in another cell to which the handover restriction is not set.
In addition, since a mobile station in communication takes the base transceiver station under handover restriction as a candidate for communication, namely handover connection, there are some cases in which the mobile station issues a handover request. However, it is general not to execute a process for the handover request to the mobile station, because the base transceiver station that has received the handover request is a base transceiver station under construction. As a result, the communication state is generally allowed to be continued without a handover.
However, when the mobile station further moves and enters deeply in a cell in which the base transceiver station under handover restriction covers, there may be some cases where the quality in communication deteriorates to cause consequent disconnection of the communication.
JP 2000-232669 A proposes a technique, by which the handover restriction is performed to a particular base transceiver station to ensure the maintenance.
JP 2000-232669 A discloses the technique, which specifies a base transceiver station for a destination cell to which the mobile station moves due to handover, on the basis of a radio state periodical report notified from a mobile station to a base transceiver station, so as to ensure that the base transceiver station under handover restriction to be in a maintainable state.
The base transceiver station for the moving destination as a result of the handover is specified based on the radio state periodically reported from the mobile station. JP 2000-232669 A discloses that it is possible to stop communication with the base transceiver station under handover restriction without having effect on a mobile communication system, and to keep the base transceiver station in a maintainable state with certainty.
In addition, various handover controlling methods have been proposed to maintain qualities in communication. For example, there has been proposed a handover controlling method which permits the handover of a mobile station in communication with a base transceiver station in a congestion state in order to ensure the qualities of communication services desired by a user, when the base transceiver station is in a congestion state (see JP 2001-251658 A).
According to the proposal of JP 2001-251658 A, a mobile communication system is established by providing a base transceiver station having the strongest electrical field intensity and a selector which selects a mobile station having observed the base transceiver station, excluding a combination of a base transceiver station currently in communication with a mobile station, as a mobile station to be an object of handover and a base transceiver station to be in communication with the mobile station.
Furthermore, there has been a technique which aims to decrease frequency of handover, thereby enhancing communication qualities and decreasing power consumption.
According to the proposal of JP 2002-27519 A, order of priority is given to multiple base transceiver stations to be a candidate of a handover destination. The reception quality is confirmed in descending order of the priority as to multiple candidate base transceiver stations. If there is any base transceiver station satisfying a predetermined condition, such a base transceiver station is determined as the handover destination.
Moreover, there has been proposed a technique, which prevents calls from mobile stations located in a cell area of a base transceiver station from being disconnected even on an occurrence of a fault in the base transceiver station.
JP 2003-78938 A discloses a technique in which a base station controller, when any of base transceiver stations under control becomes faulty, instructs all of the base transceiver stations which manage peripheral cells adjacent to the cell of the faulty base transceiver station to increase downlink transmission power. The base station controller then transmits an instruction of handover to a mobile station whose call is connected to the faulty base transceiver station. The base station controller further instructs the faulty base transceiver station to restart when the handover of the mobile station having been completed is confirmed. When the faulty base transceiver station having been recovered to the normal operation state is confirmed, the base station controller instructs the base transceiver stations which manage the peripheral cells to restore the transmission power to an original state.
There has been proposed another technique which prevents a handover request from being given to a cell with a high load (see JP 2006-5502 A).
According to the proposal of JP 2006-5502 A, a control station transmits a state of the load to a corresponding base transceiver station whenever the load of the cell is updated. The base transceiver station transfers the state of the load to a mobile station. The mobile station extracts a cell as a candidate for handover destination, on the basis of a reception level of a signal transmitted from the base transceiver station and the state of the load.
Furthermore, there has been proposed a technique which aims to perform connection to an optimal base transceiver station according to a change in circumstances.
According to the proposal of JP 2007-13463 A, an upper station manages one or more base transceiver stations to be a candidate for handover destination and its relating mobile station. The quality of a radio channel between the mobile station and the base transceiver station to be a candidate for handover destination is measured, and then the handover is executed at a timing based on a result of the measurement.
As described above, JP 2000-232669 A discloses the technique whose main purpose is to surely prevent the handover connection to the cell of the base transceiver station under handover restriction so that the implementation of the maintenance and on the like is not interfered.
In JP 2000-232669 A, however, there is no recognition on a technical problem from a viewpoint of allowing a mobile station, being used by a general user for telephone call, to maintain the telephone call surely, without using the use-prohibited cell under handover restriction. Therefore, JP2000-232669A fails to propose a solution for the problem.
In addition, the other techniques discussed above disclose techniques of appropriately performing the handover under the various conditions. However, none of the above-described techniques has proposed a radio channel controlling method, whereby the communication in the mobile station of a general user is maintained even when the mobile station of the general user encounters the use-prohibited cell under handover restriction.